1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk drive estimating a sinusoidal error in a wedge time period due to eccentricity in disk rotation.
2. Description of the Prior Art
A disk drive typically comprises one or more disks rotated by a spindle motor while heads are actuated radially over the disk surfaces. Each disk surface comprises a number of radially spaced, concentric tracks, where each track is divided into a number of data sectors. A number of embedded servo sectors forming servo wedges are also written on each disk surface, which facilitate seeking the head and maintaining the head over the centerline of a target track during read and write operations. The disks are rotated at a constant angular velocity (CAV) while varying the data rate from an inner diameter zone to an outer diameter zone to maximize the recording density.
Each servo sector comprises a sync mark for synchronizing to servo data recorded in the servo sector, such as a Gray coded track address. A sync mark detection window is opened when the head reaches the expected circumferential location for a servo sync mark as determined from a wedge period counter clocked at a predetermined frequency. Since the disk is rotated at a constant angular velocity, the sync mark detection window should be opened at a constant wedge time period (WTP). However, eccentricities in the disk rotating will introduce a sinusoidal disturbance in the WTP. Eccentricities may occur, for example, if a media writer is used to servo write the disk before installing the disk into the disk drive, if the disk “slips” after using the head internal to the disk drive to servo write the disk, or if the disk slips after writing user data to the data sectors.
U.S. Patent Application No. 2003/0184906 suggests a technique for estimating the sinusoidal disturbance in the WTP by computing a single-point Discrete Fourier Transform (DFT) to generate eccentricity compensation values used to adjust the sync mark detection window. However, using a single-point DFT to estimate the sinusoidal disturbance may require a significant number of revolutions in order to generate an accurate estimate depending on the magnitude and character of the signal noise. This estimation delay is compounded due to the sinusoidal disturbance varying over the radius of the disk requiring the estimation technique be repeated at multiple radial locations.
There is, therefore, a need to improve the technique for estimating the sinusoidal disturbance in the WTP due to eccentricities in the rotation of the disk.